Libclang tutorial

The C Interface to Clang provides a relatively small API that exposes facilities for parsing source code into an abstract syntax tree (AST), loading already-parsed ASTs, traversing the AST, associating physical source locations with elements within the AST, and other facilities that support Clang-based development tools. This C interface to Clang will never provide all of the information representation stored in Clang’s C++ AST, nor should it: the intent is to maintain an API that is relatively stable from one release to the next, providing only the basic functionality needed to support development tools. The entire C interface of libclang is available in the file Index.h

Essential types overview

All types of libclang are prefixed with CX

CXIndex

An Index that consists of a set of translation units that would typically be linked together into an executable or library.

CXTranslationUnit

A single translation unit, which resides in an index.

CXCursor

A cursor representing a pointer to some element in the abstract syntax tree of a translation unit.

Code example

// file.cpp
struct foo{
  int bar;
  int* bar_pointer;
};

#include <clang-c/Index.h>
#include <iostream>

int main(){
  CXIndex index = clang_createIndex(0, 0); //Create index
  CXTranslationUnit unit = clang_parseTranslationUnit(
    index,
    "file.cpp", nullptr, 0,
    nullptr, 0,
    CXTranslationUnit_None); //Parse "file.cpp"


  if (unit == nullptr){
    std::cerr << "Unable to parse translation unit. Quitting.\n";
    return 0;
  }
  CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit
}

Visiting elements of an AST

The elements of an AST can be recursively visited with pre-order traversal with clang_visitChildren.

clang_visitChildren(
  cursor, //Root cursor
  [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

    CXString current_display_name = clang_getCursorDisplayName(current_cursor);
    //Allocate a CXString representing the name of the current cursor

    std::cout << "Visiting element " << clang_getCString(current_display_name) << "\n";
    //Print the char* value of current_display_name

    clang_disposeString(current_display_name);
    //Since clang_getCursorDisplayName allocates a new CXString, it must be freed. This applies
    //to all functions returning a CXString

    return CXChildVisit_Recurse;


  }, //CXCursorVisitor: a function pointer
  nullptr //client_data
  );

The return value of CXCursorVisitor, the callable argument of clang_visitChildren, can return one of the three:

1. CXChildVisit_Break: Terminates the cursor traversal

2. CXChildVisit_Continue: Continues the cursor traversal with the next sibling of the cursor just visited, without visiting its children.

3. CXChildVisit_Recurse: Recursively traverse the children of this cursor, using the same visitor and client data

The expected output of that program is

Visiting element foo
Visiting element bar
Visiting element bar_pointer

Extracting information from a Cursor

Extracting the Cursor kind

CXCursorKind clang_getCursorKind(CXCursor) Describes the kind of entity that a cursor refers to. Example values:

• CXCursor_StructDecl: A C or C++ struct.

• CXCursor_FieldDecl: A field in a struct, union, or C++ class.

• CXCursor_CallExpr: An expression that calls a function.

Extracting the Cursor type

CXType clang_getCursorType(CXCursor): Retrieve the type of a CXCursor (if any).

A CXType represents a complete C++ type, including qualifiers and pointers. It has a member field CXTypeKind kind and additional opaque data.

Example values for CXTypeKind kind

• CXType_Invalid: Represents an invalid type (e.g., where no type is available)

• CXType_Pointer: A pointer to another type

• CXType_Int: Regular int

• CXType_Elaborated: Represents a type that was referred to using an elaborated type keyword e.g. struct S, or via a qualified name, e.g., N::M::type, or both.

Any CXTypeKind can be converted to a CXString using clang_getTypeKindSpelling(CXTypeKind).

A CXType holds additional necessary opaque type info, such as:

• Which struct was referred to?

• What type is the pointer pointing to?

• Qualifiers (e.g. const, volatile)?

Qualifiers of a CXType can be queried with:

• clang_isConstQualifiedType(CXType) to check for const

• clang_isRestrictQualifiedType(CXType) to check for restrict

• clang_isVolatileQualifiedType(CXType) to check for volatile

Code example

//structs.cpp
struct A{
  int value;
};
struct B{
  int value;
  A struct_value;
};

#include <clang-c/Index.h>
#include <iostream>

int main(){
  CXIndex index = clang_createIndex(0, 0); //Create index
  CXTranslationUnit unit = clang_parseTranslationUnit(
    index,
    "structs.cpp", nullptr, 0,
    nullptr, 0,
    CXTranslationUnit_None); //Parse "structs.cpp"

  if (unit == nullptr){
    std::cerr << "Unable to parse translation unit. Quitting.\n";
    return 0;
  }
  CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit

  clang_visitChildren(
  cursor,
  [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){
    CXType cursor_type = clang_getCursorType(current_cursor);

    CXString type_kind_spelling = clang_getTypeKindSpelling(cursor_type.kind);
    std::cout << "Type Kind: " << clang_getCString(type_kind_spelling);
    clang_disposeString(type_kind_spelling);

    if(cursor_type.kind == CXType_Pointer ||                     // If cursor_type is a pointer
      cursor_type.kind == CXType_LValueReference ||              // or an LValue Reference (&)
      cursor_type.kind == CXType_RValueReference){               // or an RValue Reference (&&),
      CXType pointed_to_type = clang_getPointeeType(cursor_type);// retrieve the pointed-to type

      CXString pointed_to_type_spelling = clang_getTypeSpelling(pointed_to_type);     // Spell out the entire
      std::cout << "pointing to type: " << clang_getCString(pointed_to_type_spelling);// pointed-to type
      clang_disposeString(pointed_to_type_spelling);
    }
    else if(cursor_type.kind == CXType_Record){
      CXString type_spelling = clang_getTypeSpelling(cursor_type);
      std::cout <<  ", namely " << clang_getCString(type_spelling);
      clang_disposeString(type_spelling);
    }
    std::cout << "\n";
    return CXChildVisit_Recurse;
  },
  nullptr
  );

The expected output of program is:

Type Kind: Record, namely A
Type Kind: Int
Type Kind: Record, namely B
Type Kind: Int
Type Kind: Record, namely A
Type Kind: Record, namely A

Reiterating the difference between CXType and CXTypeKind: For an example

const char* __restrict__ variable;

• Type Kind will be: CXType_Pointer spelled "Pointer"

• Type will be a complex CXType structure, spelled "const char* __restrict__

Retrieving source locations

CXSourceRange clang_getCursorExtent(CXCursor) returns a CXSourceRange, representing a half-open range in the source code.

Use clang_getRangeStart(CXSourceRange) and clang_getRangeEnd(CXSourceRange) to retrieve the starting and end CXSourceLocation from a source range, respectively.

Given a CXSourceLocation, use clang_getExpansionLocation to retrieve file, line and column of a source location.

Code example

// Again, file.cpp
struct foo{
  int bar;
  int* bar_pointer;
};

clang_visitChildren(
  cursor,
  [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

    CXType cursor_type = clang_getCursorType(current_cursor);
    CXString cursor_spelling = clang_getCursorSpelling(current_cursor);
    CXSourceRange cursor_range = clang_getCursorExtent(current_cursor);
    std::cout << "Cursor " << clang_getCString(cursor_spelling);

    CXFile file;
    unsigned start_line, start_column, start_offset;
    unsigned end_line, end_column, end_offset;

    clang_getExpansionLocation(clang_getRangeStart(cursor_range), &file, &start_line, &start_column, &start_offset);
    clang_getExpansionLocation(clang_getRangeEnd  (cursor_range), &file, &end_line  , &end_column  , &end_offset);
    std::cout << " spanning lines " << start_line << " to " << end_line;
    clang_disposeString(cursor_spelling);

    std::cout << "\n";
    return CXChildVisit_Recurse;
  },
  nullptr
);

The expected output of this program is:

Cursor foo spanning lines 2 to 5
Cursor bar spanning lines 3 to 3
Cursor bar_pointer spanning lines 4 to 4

Complete example code

#include <clang-c/Index.h>
#include <iostream>

int main(){
  CXIndex index = clang_createIndex(0, 0); //Create index
  CXTranslationUnit unit = clang_parseTranslationUnit(
    index,
    "file.cpp", nullptr, 0,
    nullptr, 0,
    CXTranslationUnit_None); //Parse "file.cpp"

  if (unit == nullptr){
    std::cerr << "Unable to parse translation unit. Quitting.\n";
    return 0;
  }
  CXCursor cursor = clang_getTranslationUnitCursor(unit); //Obtain a cursor at the root of the translation unit


  clang_visitChildren(
  cursor,
  [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){
    CXType cursor_type = clang_getCursorType(current_cursor);

    CXString type_kind_spelling = clang_getTypeKindSpelling(cursor_type.kind);
    std::cout << "TypeKind: " << clang_getCString(type_kind_spelling);
    clang_disposeString(type_kind_spelling);

    if(cursor_type.kind == CXType_Pointer ||                     // If cursor_type is a pointer
      cursor_type.kind == CXType_LValueReference ||              // or an LValue Reference (&)
      cursor_type.kind == CXType_RValueReference){               // or an RValue Reference (&&),
      CXType pointed_to_type = clang_getPointeeType(cursor_type);// retrieve the pointed-to type

      CXString pointed_to_type_spelling = clang_getTypeSpelling(pointed_to_type);     // Spell out the entire
      std::cout << "pointing to type: " << clang_getCString(pointed_to_type_spelling);// pointed-to type
      clang_disposeString(pointed_to_type_spelling);
    }
    else if(cursor_type.kind == CXType_Record){
      CXString type_spelling = clang_getTypeSpelling(cursor_type);
      std::cout <<  ", namely " << clang_getCString(type_spelling);
      clang_disposeString(type_spelling);
    }
    std::cout << "\n";
    return CXChildVisit_Recurse;
  },
  nullptr
  );


  clang_visitChildren(
  cursor,
  [](CXCursor current_cursor, CXCursor parent, CXClientData client_data){

    CXType cursor_type = clang_getCursorType(current_cursor);
    CXString cursor_spelling = clang_getCursorSpelling(current_cursor);
    CXSourceRange cursor_range = clang_getCursorExtent(current_cursor);
    std::cout << "Cursor " << clang_getCString(cursor_spelling);

    CXFile file;
    unsigned start_line, start_column, start_offset;
    unsigned end_line, end_column, end_offset;

    clang_getExpansionLocation(clang_getRangeStart(cursor_range), &file, &start_line, &start_column, &start_offset);
    clang_getExpansionLocation(clang_getRangeEnd  (cursor_range), &file, &end_line  , &end_column  , &end_offset);
    std::cout << " spanning lines " << start_line << " to " << end_line;
    clang_disposeString(cursor_spelling);

    std::cout << "\n";
    return CXChildVisit_Recurse;
  },
  nullptr
  );
}